3 Rom Switcher

[gwpt]

After spending the last few days modding my ZX81, I got inspired to make a mod of my own
So I thought, it would be nice to be able to have all 3 popular roms - the standard, the improved and the "shoulders of giants" all available at the flick of a switch.

So I made a little daughter board which sits in the ROM socket.
disclaimers:
* It didn't quite fit, so I did need to file away a bit of the case Not Ideal
* Compared to the included photos, I reworked it so there are 2 less rows of stripboard at the bottom, this should reduce how much it touches the case and mean less filling. I haven't built this version, but it will be ok, it passed all the tests. The plans are of the updated version
* I like to socket chip but it was too tall, so I had to solder the EPROM directly to the board, so no update!
* its still a fraction too tall, but the case does screw together ok with a tiny bit of pressure

Operation is as follows:
A0-A12 are wired as a normal EPROM would be.
A13 and A14 are connected to the switch with 2 pulldown resistors.
So in the centre position (The switch isn't connected at all), A13 and A14 are low
When the switch is on in either position, either A13 or A14 go high.
This then maps to these parts of the EPROM: 0-8k = improved, 8K-16k = standard, 16K-24k giants. You need to burn a 27C256 EPROM

Here's the schematic:

and here are the stripboard layouts:

This is where to cut the stripboard:

Bottom.png (8.07 KiB) Viewed 3853 times

And if you're interested here are the TinyCad and VeeCad source files:
https://www.dropbox.com/scl/fi/grz2tv8w ... t=19s1mddg
https://www.dropbox.com/scl/fi/xq2ugbr3 ... t=gh8lbz85

the 2 resistors are 10k
and the switch is an ON-OFF-ON 3 postition switch.

Here is the rom image for the 3 combined firmwares.
https://www.dropbox.com/scl/fi/hr3e7a92 ... t=16ixrp23

and finally some pics, again please note, this is my v1 which has 2 extra row at the bottom, in v2 (which is included here) I've shifted them to the top:

[Moggy]

What a lovely thing to behold! All done in the spirit of strip board modders everywhere, especially the bit about filing the case so things would fit,just the job.

I have EPROMs fitted to my zeddies and as you've discovered headroom is at a premium. A forum member made up a few specials for me using those little square chips that are usually pushed into those small square surrounds as found on PC motherboards and were far too tall, we live and learn I suppose

Yes very nice indeed and equally heart warming is that it's being modded to be used and not put in a collectors cabinet.
I'm surprised you didn't plump for the double speed BASIC, "Big Bang" ROM code though as it really does help BASIC chug along quite nicely.

More power to your iron sir!

[gwpt]

Thanks moggy!

I do have a soft spot for stripboard. It’s so quick and easy to whip stuff up (especially with veecad, which very pleasantly is now free!

I'm surprised you didn't plump for the double speed BASIC, "Big Bang" ROM code though as it really does help BASIC chug along quite nicely.

Argh, being new to the zx81 i had only heard of the 3 roms I put on it…
I might redo it with my v2 as it would fit more nicely.

So if you had to pick 3 roms, which 3 would you pick?

[1024MAK]

Nice

By the way, you can attach pictures and ZIP files to posts.

Out of interest, and me being nosey, which other mods did you do?

Mark

[1024MAK]

Also note that there are three versions of the official Sinclair ROM...

I would pick:

• The last official bug fixed Sinclair ROM
• The shoulders of giants ROM
• Big Bang ROM

Mark

[gwpt]

Thanks Mark

Oh ok, I managed to attach pictures, but I got a warning when I tried the bin file. I didn’t try zip..

Mods so far:
* video mod - the 555 one but Ill remove it when I get the vLA81
* my ribbon cable broke so - shorten ribbon cable, removed the connectors and attached them with wires (i glue gunned the connectors to the case, so the remaining ribbon wouldnt move)
* 32k ram mod suggested in the vLA81 docs
* not really a mod but ai built a tzxduino

Mods in the mail:
* vLA81
* esp32 to make a video wespi. I was going to put it in an external box and put a connector on the back of the zx
* switched(?) regulator instead of 7805
* i found a candidate for a barrel power socket! Bit of a long shot but worth a try

As you can probably see, I get a bit mod obsessed with things!

Thanks for the rom suggestions. I don’t suppose you know where I could get a copy of the final version from sinclair and the bing bang Rom? I cant seem to find them…

Perhaps, I should change the toggle switch to a 4 position rotary, and use 3 diodes to encode the positions down to 2 bits, then I could have 4 roms! ( I included the ‘standard’ as I had read that there are a few things that only work with the sqrt bug.)

[Moggy]

Big Bang thread here..

https://www.sinclairzxworld.com/viewtop ... g+bang+rom

The ROMs.

For the Big Bang use the one labeled "zx81x2" the other one "zx81x2c" is for something else.

My three.

1) Big Bang.

2) H4th

3) 649 Sinclair final.

[gwpt]

Cheers Moggy!

[Moggy]

I have edited post to reflect which version of Big bang to use and listed my ROM preferences.

[gwpt]

i was just reading up on h4th. Looks very interesting. I’ve never tried forth so might be fun to have a go.
I found a few other posts, and it seems there are many alternate roms,,eg aszmix, tree forth, etc
Maybe I should make a 8/16 way switch/selector
Do you know where I could find h4th, or in fact all the roms? I seem to be struggling to find files on the forum